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Flow of gases through polyethylene



Diffusion and permeability constants for twelve permanent gases have been measured in a linear and a branched polyethylene, hydrogenated polybutadiene, and natural rubber over the temperature range 5–55°C. The Barrer time lag apparatus has been used in these determinations. For all gas–polymer pairs investigated, a linear plot of the logarithm of the diffusion or permeability constants versus the reciprocal absolute temperature is observed, indicating activated diffusion is occurring. Arguments have been presented supporting the hypothesis that natural rubber is a completely amorphous analog of polyethylene with respect to the diffusion process. It has been possible, as a result, to quantitatively express the reduction in diffusion constants in going from natural rubber to hydrogenated polybutadiene (29% crystalline), to the branched polyethylene (43% crystalline), and to the linear polyethylene (77% crystalline). These reductions were strongly dependent on gas molecular size, increasing as the molecular size increases. A geometric impedance factor and a chain immobilization factor have been introduced to account for the effect of crystallinity. The former is assumed to be independent of molecular size and accounts for the necessity of the diffusing molecule to by-pass crystallites which have been shown to be impenetrable. The latter size-dependent factor reflects the reduction in amorphous chain segment mobility brought about through the proximity of crystallites. Analysis of the geometric impedance factor supports the existence of thin, highly anisometric sheets of crystalline polymer in polyethylene. The anisometry of the crystallites was observed to increase with increasing crystallinity consistent with the chain folding mechanism for crystal growth in polyethylene. Nucleation and growth kinetics also qualitatively accounted for the variation in crystal anisometry observed in polymers prepared by different methods of polymerization. The apparent activation energies for diffusion determined in polyethylene include not only the energy required for chain segment separation but also the effect of thermal expansion and crystalline melting. Correlations have been presented which permit the estimation of gas diffusion constants in a wide variety of polyethylenes.

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